This is generated in the source directory by meson when the project is checked
out of git by the meson wrap system.  Ignoring it here ignores issues with
tools that assume a clean source directory, like the REUSE test.

Fedora takes issue with CC0.  Although it doesn't really matter for this
stuff (dual licensed anyway, questionably "software", certainly not patentable,
and so on), this is simpler and more consistent with the ISC license used for
the actual software anyway.